Magnetic tape cartridges are often used to backup computer hard drives and disk arrays nightly, and to provide mass storage for information that is not accessed on a regular basis. For standalone computers and small network systems, a single manually loaded tape drive is sufficient to handle most backup and archived information needs. As the storage requirements expand, however, it becomes practical to integrate the tape drive into an automated library that can mount and dismount one of several tape cartridges under computer control.
Small to medium office network systems can benefit from a small desktop or rack-mounted library having multiple tape cartridges. Current magnetic tape cartridges, such as the digital linear tape (DLT) and eight-millimeter formats, have capacities greater than twenty gigabytes per cartridge. These capacities are sufficient to copy the entire contents of modest disk servers on just one cartridge. By providing seven or more cartridges in the library, backup operations can be performed automatically every day for an entire week.
The capacity of rack-mounted libraries is usually limited by the standard nineteen-inch width of the rack, the need for a user to access the cartridges, and the desire for the user to be able to see the labels on the cartridges. Many libraries are configured with all of the cartridges and drives lined up across the front of the housing with the cartridge labels facing outward. A robotic mechanism transports the cartridges to and from the drives from the label side of the cartridges. Operating on the label side of the cartridges also allows the robotic mechanism to carry a bar-code reader that can scan bar-codes on the labels for positive cartridge identification. The advantages of these configurations are easy user access to the cartridges, a direct user and robotic mechanism view of the labels to determine quickly which cartridges are installed, a simple robotic mechanism, and a low housing height determined mainly by the height of the cartridges. The primary disadvantage is that only a limited number of cartridges and drives fit across the nineteen-inch rack width. Using the DLT cartridges as an example, only seven DLT cartridges and one DLT media drive fit within the nineteen-inch width constraint.
To increase the total capacity of the library, some library configurations position the drives in a back row behind the cartridges. Front row space vacated by the drives is used to hold additional cartridges. The robotic mechanism is now positioned between the cartridges and drives. Using the DLT cartridge example again, approximately eleven DLT cartridges can be positioned across the front row of the housing and up to three DLT media drives can be positioned in the back row. The primary limitation of these configurations is still the total number of cartridges that can be installed at any given time. A bar-code reader requirement acts to reduce the total number of cartridges. Since the robotic mechanism cannot view the bar-codes on the labels directly, the bar-code reader cannot be mounted on the robotic mechanism. Instead, the bar-code reader is installed facing inward in one cartridge position. Here the bar-code reader scans the cartridge held in front of it by the robotic mechanism. The tradeoff for the bar-code reader capability is one less cartridge in the library.
Two other configurations have been used to increase the total number of cartridges within the library. Some configurations place additional cartridges in the back row next to the drives. An example of this configuration can be seen in U.S. Pat. No. 5,892,750, issued to Barkley et al., on Apr. 6, 1999. Barkley discloses a library having ten cartridges held in a removable magazine positioned at the front of the housing. Five additional cartridges are stored in a back row next to the drive. These five additional cartridges cannot be directly accessed by a user, nor can they be directly viewed by a user who wishes to read their labels. Another library configuration stacks a second row of cartridges above the first row at the front of the housing. Here, all of the cartridges are readily accessible and viewable to the user. The tradeoffs for having vertically stacked cartridges are that the height of the housing is doubled, and this library requires a more complicated robotic mechanism that can move up and down as well as left and right.
Yet another configuration is disclosed in U.S. Pat. No. 5,870,245 issued to Kersey et al., on Feb. 9, 1999. Kersey discloses a library where the cartridges and drives are aligned in a row perpendicular to the front of the housing. In this approach, the total number of cartridges and drives are limited by the depth of the housing instead of by the width of the housing. Since the housing can be greater than nineteen inches deep, more distance is available to arrange the cartridges and drives. One limitation of the Kersey disclosure is that all of the cartridges are hidden from the user when installed in the library. In order to access or view the label of any one cartridge, a magazine holding all of the cartridges must be removed from the library. Removing the magazine from the library causes all cartridge transfers to and from the magazine to be halted until the magazine is reinstalled.
To increase the capacity of rack-mounted libraries, more efficient placements of the cartridge, drive and robotic mechanism are required. What is desired is a configuration in which the height of the housing is kept to a minimum, all of the cartridges are directly accessible by the user, and all of the labels are directly viewable by the user and the robotic mechanism.